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Practical experience using Verti-Fan 2009

Practical experience using Verti-Fan in various crops.

The Verti-Fan vertical ventilation system creates a better greenhouse climate.
The Verti-Fan consists of a fan mounted in the greenhouse by means of stainless steel supports on the underside of the trellis girder. If a screen is installed, the fan is mounted on a longer support, 30 cm below the screen. Plastic trunking 38 cm in diameter is fastened to the fan and hangs down vertically to 40 cm above the surface of the ground. A circular or oval stainless steel ring is fitted to the end of the trunking to make it stable and keep the air outlet open. The oval model leaves more space in the path for passing tubular rail carts.
The fan has been designed especially for blowing air through trunking with minimum loss of capacity when mounted vertically.
The Verti-Fan is intended for creating vertical air currents without causing crop movement. The air leaving the trunking disperses over the ground and rises up through the crop.
One Verti-Fan is installed for approximately every 450 m² of greenhouse when fruit-vegetables are cultivated on channels suspended at least 50 cm aboveground. The fan has a capacity of 3500 m³ per hour.
The Verti-Fan is connected to a panel with a speed control, so that the fan’s capacity can be reduced, if necessary, for smaller and more vulnerable crops.

In summary, the advantages of the Verti-Fan system are:

  • Minor horizontal and vertical temperature differences.
  • Better distribution and uptake of CO2.
  • Better control of humidity.
  • No stationary air at the bottom of the crop.
  • Energy-saving thanks to a lower minimum heating-pipe temperature
  • Window vents can remain closed for longer.
  • Temperature reduction during warm evenings and nights.
  • Better crop and heavier fruit
  • Fewer fungal problems.

Minor horizontal and vertical temperature differences. Hot air at the top of the greenhouse caused by external radiation and lighting is transferred by the Verti-Fan to the bottom of the greenhouse. Heat produced by external radiation near walls is also dispersed. This means there are fewer horizontal and vertical temperature differences in the greenhouse. Therefore crop growth is more uniform and harvesting starts at approximately the same time everywhere in the greenhouse. In a 3-hectare greenhouse with paths 80 metres long, the difference in leaf temperature between the middle path and the wall was 5ºC. After installing the Verti-Fan the difference fell to 1ºC. In a 1-hectare greenhouse used for growing tomatoes there was a temperature difference of 6ºC. Lighting and a Verti-Fan were installed, after which the temperature difference was less than 1ºC, even though lighting increases temperature differences.
When the outside temperature is low, the Verti-Fan draws in the cool air at the top of the greenhouse and mixes it with the warmer greenhouse air below. This reduces the risk of cold plant tops.

Better distribution and uptake of CO2.
If there is little or no air current between the leaves, the CO2 concentration close to the leaf surface drops, owing to the uptake of CO2 and the limited supply. With the Verti-Fan, the rising air ensures better dispersal of CO2 between the leaves, thereby increasing the plant’s CO2 uptake. The circulating air also disperses CO2 better throughout the entire greenhouse.

Better control of humidity.
Because the warm, dryer air is transported downward, relative humidity at the bottom of the crop can decrease (as warm air can absorb more moisture than cold air). Condensation on the crop or fruit therefore occurs less often.
At a tomato grower’s with a fixed greenhouse plastic film, at an outside temperature 1oC, the temperature at the bottom of the greenhouse was 2.2oC lower and the humidity 6% higher than at the top. Two hours after switching on the Verti-Fan, the temperature at the bottom was 0.3oC lower and the humidity was 1% higher than at the top. Better air displacement also enables the humid air to condense better against the glass, thereby decreasing the humidity.

No stationary air at the bottom of the crop.
The Verti-Fan blows the air to the bottom of the crop. The air then disperses horizontally over the soil and rises up through the crop again slowly. This air is frequently warmer, promotes evaporation and keeps the crop more active.
Energy-saving thanks to a lower minimum heating-pipe temperature.
The minimum heating-pipe temperature is usually necessary to activate the crop and control humidity. The air rising slowly from the Verti-Fan through the crop partially takes over this task. A lower minimum heating-pipe temperature will therefore often suffice. The resulting energy-saving is around 5% in practice.

Window vents can remain closed for longer.
Drawing down the warm air from the top of the greenhouse makes it less necessary to remove heat through vents. Better humidity control also means that window vents can stay closed for longer or that the window need not be open so wide. This can also slightly increase the CO2 concentration.
Temperature reduction during warm evenings and nights.
In summer, inland areas of the country sometimes have hot days followed by windless evenings when the greenhouse temperature remains high for much too long. The cooler air outside can be drawn in using the Verti-Fan. This prevents crop “scorching” in hot periods. For aubergine (eggplant) cultivation, the temperature at the bottom of the crop has to be brought down quickly at sundown. This can be done in 1.5 hours using the Verti-Fan.

Better crop and heavier fruit.
When the Verti-Fan is used, the even temperatures result in a more uniform crop. The better air movement combined with high CO2 uptake benefits the crop. In the case of tomatoes, the plants remained 1-2% shorter, the leaves 5% longer and the top was also 3-4% thicker. Aubergine leaves were also darker and the plant tops were somewhat fresher and greener at the end of cultivation. Aubergine fruit was 20-30 grams heavier than fruit of the same size in a department with no Verti-Fan.

Fewer fungal problems.
The Verti-Fan produces good air currents through the crops, thereby preventing local areas of excessive humidity and reducing the likelihood of the crop being damaged by fungal growth. Tomato plants were found to have fewer Botrytis patches on the stem.

The following points are important when using the Verti-Fan

Experience has shown that the fans have to be on for around 1.5 hours to achieve proper mixing of greenhouse air. Consequently, switching on and off for short periods is pointless.

Be careful when blowing cold air downwards.
If cold air is drawn in through window vents or from above an energy screen and blown downwards, the bottom of the crop may become too cold. If this air is much colder than the minimum room temperature, the crop and the substrate will cool down too much, which is detrimental for crop growth. Condensation may occur on stems and fruit if the room temperature rises too quickly.

If the crop reacts differently, adjustment may be necessary.
The improved air current through the crop can promote evaporation as well as growth. It may therefore be necessary to maintain the right balance in the crop by increasing the room temperature or the minimum heating-pipe temperature.

Air currents can spread diseases.
Using the Verti-Fan can limit the occurrence of fungal diseases. However, if fungal diseases occur in the crop, the air currents make it easier for fungal spores to spread through the air. In such a situation the fans should be switched off.

Verti-Fan is not a dehumidification system.
The humidity outside is sometimes high in late summer. Displacing the humid air through the Verti-Fan to the bottom of the crop can lead to fungal growth. Humidity can be reduced by maintaining the proper minimum heating-pipe temperature.

Krijn Buitelaar
Cultivation adviser